Method for producing superalloy articles by hot isostatic pressing

ABSTRACT

THE PRODUCTION OF SUPERALLOY ARTICLES FROM SUPERALLOY POWDERS CONTAINING A DISPERSION OF GAMMA PRIME PARTICLES AND AN AVERAGE GRAIN DIAMETER OF 20 MICRONS MAXIMUM BY HEATING THE POWDER TO A TEMPERATURE AT OR ABOVE A SELECTED COMPACTING TEMPERATURE BUT BELOW THE TEMPERATURE AT WHICH THE GAMMA PRIME PARTICLES DISSOLVE AND ISOSTICALLY COMPACTING THE POWDER TO A DENSITY OF AT LEAST 95% PRIOR TO COOLING BELOW THE SELECTED COMPACTING TEMPERATURE.

Oct. 17, 1972 s K ETAL METHOD FOR PRODUCING SUPERALLOY ARTICLES BY HOT ISOSTATIC PRESSING Filed April 30, 1971 GAMMA PRIME 50L VU$ 7' E MPE RA T U/PE ASTROLO) llV-IOO HEA TING TEMPERATURE PF) wmvroes AUGUST KASAK, VERNON R THOMPSON 8 JOHN h.

Attorney United States Patent C 3,698,962 METHOD FOR PRODUCING SUPERALLOY ARTICLES BY HOT llSOSTATIC PRESSIING August Kasak, Upper St. Clair, Vernon R. Thompson, Greentree Borough, Allegheny County, and John H. Moll, Ross Township, Allegheny County, Pa., assignors to Crucible Inc., Pittsburgh, Pa.

Filed Apr. 30, 1971, Ser. No. 139,126 Int. Cl. B22f 9/00; C22f 1/00 US. Cl. 14811.5 F Claims ABSTRACT OF THE DISCLOSURE It is known to produce articles such as engine components, dies and the like from so-called superalloys which typically may be nickelor cobalt-base alloys. In the current commercial practice for superalloys, and particularly nickel-base superalloys, such are melted, usually in a vacuum, solidified in ingot form, and hot worked by rolling and/or forging to the desired final product configuration, which working operations generally involve a multiplicity of steps. After working the product is heat treated by solution annealing and age-hardening treatments. In these treatments the strength and ductility characteristics are controlled both by the extent of deformation and the temperature employed both during working and subsequent heat treatment. It is generally accepted that fine grain size is necessary in many applications, such as turbine discs to achieve the desired high yield strength in typical broad service temperature ranges, which may be from subzero to about 1400 F. In other applications, however, such as turbine blades, larger grain sizes in the final products are desired to impart the required creep and creep-rupture strength during service. In any event, however, it is desirable to obtain in all product forms a uniform grain size throughout the cross section of the part and particularly the load-bearing cross section. This is extremely difiicult if not impossible to obtain with the above-described current commercial practices. It is necessary with superalloys for these applications to subject the same to a forging operation prior to production of final articles. Because of the high alloy content it is difiicult to effect forging because of inherent lack of ductility and resistance to deformation, the former being represented by percent elongation and reduction in area values and the latter being represented by the yield and tensile strength values. In addition, it is desirable that these alloys no matter what the application have a uniform grain structure throughout the cross section. Also, for many applications, the material should be characterized by a high yield strength; whereas, for other applications it is desirable that the product be characterized by high creep and creep-rupture strength despite an attendant lower yield strength.

It is accordingly the primary object of the present invention to provide a powder metallurgy process for producing superalloy articles wherein a uniform grain structure throughout the cross section of the article is achieved along with ease of initial compacting to the required high densities and subsequent hot working. In addition, the

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articles after compacting may be subjected to alternate heat-treating practices wherein either high yield strengths, or high creep and creep-rupture strengths may be achieved in the final articles.

This and other objects of the invention, as well as a complete understanding thereof, may be obtained from the following description, specific examples and drawing, the single figure of which is a graph showing the relationship between compacting temperature and grain size for two nickel-base superalloys.

Broadly, in the practice of the invention superalloy powder'of a size not'exceeding about'ininus 30 mesh is used. The powder has a characteristic dispersion of gamma prime particles, and in addition the powder has an average grain diameter not exceeding 20 microns maximum. The grain size, as represented by the average grain diameter, is dependent upon the solidification rate of the powder during the powder-making operation, which generally involves atomizing a quantity of molten metal and thereafter solidifying the resulting molten metal droplets to form the powder. The more rapid the cooling rate the finer will be the grain size of the powder. Consequently, since smaller-size powder solidifies at a more rapid rate than larger-size powder, grain size of the powder will be a function of powder size. The practice of the invention is preferably limited to minus 30 mesh maximum powder. This powder-size limitation, in turn, limits the average grain diameter to 20 microns maximum by typical powder-making practice. With respect to the dispersion of gamma prime particles, or the gamma prime phase, such constitutes a characteristic intermetallic dispersion in the base-metal matrix of the superalloy. More specifically, the microstructure of nickel-base superalloys, as well as some cobalt-base superalloys, includes (1) a continuous, solid-solution base-metal matrix (gamma phase); (2) gamma prime particles; and (3) carbides. The gamma prime particles constitute a face-centered cubic intermetallic compound with a typical, generic formula of Ni Al. The gamma prime particles of this character comprise a uniquely effective strengthening agent particularly in nickel-base and nickel-rich alloys by a precipitation hardening mechanism and are distributed throughout the base-metal matrix.

Powder of this character is placed in a container and treated to remove air from the container interior and surface impurities, such as oxides, by conventional outgassing techniques, which includes heating to a relatively low temperature on the order of 500 F. and pumping out the container interior to remove the air and any gaseous reaction products therefrom. Thereafter the particles are heated in a protective atmosphere, which may be either an inert gas or a vacuum, to a temperature at or above a selected compacting temperature. It is critical, however, that the particles not be heated above a maximum temperature constituting the gamma prime solvus temperature of the superalloy. The gamma prime solvus temperature is the minimum temperature at which the gamma prime phase is essentially completely dissolved in the matrix of the alloy by heating for not longer than one minute. The gamma prime solvus temperature will vary from alloy to alloy composition, specific examples of which for selected alloy compositions are given hereafter. Also, the minimum compacting temperature will vary from alloy to alloy composition, but is not lower than about 400 F. below the gamma prime solvus temperature for any said alloy composition. In addition, to achieve the required high densities for a product of sufiicient integrity without compacting at temperatures exceeding the gamma prime solvus temperature it is further critical that the compacting be effected isostatically as by the use of a fluid pressure vessel. These vessels, commonly termed autoclaves, achieve compacting by the introduction thereto under high pressure of a fluid, which is usually a gas such as nitrogen, helium or argon. In this manner compacting pressures are homogeneous which in the practice of the invention results in a product of the required high density, e.g. 95% or greater, without requiring that the powder to be compacted be heated above the gamma prime solvus temperature. Although the gamma prime solvus temperature will vary from alloy to alloy as stated above, it is readily determinable for a particular alloy and is generally about 1600 to 2300 F.

Generally to facilitate outgassing and subsequent heating and compacting, the powder will be placed in a container wherein it is outgassed, evacuated, sealed against the atmosphere, heated and then compacted in the autoclave. With superalloys, after compacting the article is generally subject to hot working, as by a forging operation, to various shapes depending upon the final product desired. In accordance with the invention, hot working is facilitated if the temperature at which hot working is conducted does not exceed the gamma prime solvus temperature. After hot Working if an article characterized by high yield strength is required any subsequent heating should not be at a temperature exceeding the gamma prime solvus temperature. Alternatively, however, if a product having increased creep and creep-rupture strength is desired, these values may be improved by heating the article at a temperature above the gamma prime solvus temperature.

During compacting in the autoclave to final densities it has been determined in the practice of the invention that by the application of fluid pressure in the range of 1,000 to 50,000 p.s.i. it is possible to achieve final densities of at least about 95% and typically 97% and greater. If desired, to facilitate handling and the like the powder charge may be subjected to compacting to an intermediate density prior to the final compacting to thus constitute a two-step compacting operation. In this instance, the use of a container during final compacting may possibly be avoided.

For use in specific examples demonstrating the inven tion the nickel-base superalloys listed in Table I were employed.

The above alloys were provided in powder form having a size of minus 30 mesh and were placed in hollow cylinder containers which were evacuated and sealed against the atmosphere. The containers were subjected to heating at various compacting temperatures and compacting pressures. After compacting, the compact specimens were subjected to the following heat treatments. The Waspaloy material was heated to a temperature of 1875 F. for four hours, and thereafter air cooled; heated to 1550 F. for 24 hours and thereafter air cooled; and finally heated to 1400" F. for 16 hours and then thereafter air cooled. The Astroloy compact was heated to a temperature on the order of 2025 to 2075 F. for four hours and thereafter air 4 cooled; heated to 1600 F. for eight hours and thereafter air cooled; heated to 1800 F. for four hours and thereafter air cooled; heated to 1200 F. for 24 hours and thereafter air cooled; and heated to 1400 F. for eight hours and thereafter air cooled. The IN100 compact was given no heat treatment.

The grain size of the powders used was initially very fine and on the order of about 3 microns diameter.

The Waspaloy material has an inherently lower strength or resistance to deformation at high temperature than does the IN100 material. Specifically, the 0.2% offset yield strength at 1800 F. for Waspaloy is 20,000 p.s.i. and for IN-lOO is 54,000 p.s.i. However, when these materials in the powdered form as described above were heated to a temperature of 2000 F. and compacted using an autoclave operated at a fluid pressure of 15,000 p.s.i., the IN-lOO compact achieved an essentially full density (99% of theoretical density); whereas, the Waspaloy compact exhibited appreciable porosity and a density of less than about 97%. Substantially the same result was achieved with these two materials when they were compacted at a temperature of 2100 F. In view of the significantly higher strength and resistance to deformation of the IN-100 material these compacting results were totally unexpected in that one would expect this higher strength material to, under identical conditions, not achieve as great a density as the lower strength Waspaloy material. It was determined, however, that the reason for the unexpected behavior was that the Waspaloy material was heated at a temperature above the gamma prime solvus temperature for this particular alloy; whereas, the IN 100 material was not heated above its gamma prime solvus temperature. The gamma price solvus temperature for Waspaloy is about 1850 F. and the gamma prime solvus temperature for IN-100 is about 2225 F.

Although this phenomenon is not completely understood, it is believed that the gamma prime particles, when undissolved, act as grain growth inhibitors and consequently so long as heating is not effected at a temperature level wherein the gamma prime particles are dissolved the initially fine grain diameter is preserved. By preserving this fine grain the material exhibits hitherto unobtainable and unexpected formability. In this regard, it has been determined that the grain diameter should not exceed about 20 microns maximum, and preferably 10 microns maximum.

This beneficial phenomenon it was found could also be carried over and taken advantage of during subsequent hot working. Consequently, alloys that could not be conventionally forged when produced in accordance with prior art practice may now be forged by conventional techniques when otherwise processed in accordance with the present invention.

Table II presents mechanical property data for IN-lOO compacts that had been isostatically compacted at the temperatures listed in the table. The gamma prime solvus temperature for IN-100 is about 2225 F. It may be seen from high temperature tension test results given in Table II that as compacting temperature was increased to about the gamma prime solvus temperature the alloys resistance to deformation, which is represented by the yield and tensile strength values, was increased and the alloys ductility, as represented by the percent elongation and reduction in area values, was decreased. Particularly remarkable ductility was obtained when the material that had been compacted at 2000 F. was tension tested at 1950 F.; elongation of 518% and reduction of area of 98% were measured. As is well known, low resistance to deformation and high ductility are requisite for good hot workability of an alloy. Specifically, in this regard, a compact of the IN100 material compacted at a temperature of 2100" F. was hot forged to an reduction in area while IN-IOO material conventionally produced by hot metal casting techniques and in the coarse-grain condition was found to be unworkable under identical forging conditions. The properties of the conventional material are also presented in Table II.

6 With IN-IOO compacts, as shown in Table IV, the yield strength decreased as the compacts were heated progres- As earlier mentioned, superalloys, depending upon their end use, may be required to have either high yield strength, or good creep and creep-rupture strength. These strength characteristics are conventionally controlled by the extent of deformation and temperatures used in the various working operations and by the heat treatment applied thereafter. Specifically in this regard, fine grain size is necessary to obtain a product that exhibits a high yield strength; larger grain sizes are advantageous in applications wherein creep and creep-rupture strength are significant. For IN-lOO and Astroloy alloys of Table I 25 the figure shows graphically the grain diameter as affected by temperature and specifically the gamma prime solvus temperature for each of the alloys. It may be seen from the figure and from the data presented in Tables III and IV that grain size increases drastically upon heating to temperatures above the gamma prime solvus temperature. Consequently, the final product which is characterized by high yield strength and ductility characteristic of the fine grain size imparted by processing in accordance with the present invention may if desired have creep and creeprupture strength increased by final heat treatment at temperatures above the gamma prime solvus temperatures.

TABLE TIL-ASTROLOY Tensile properties at room temperature Grain Reducsize Elongation of Compact diam Yield Tensile tion area temperature eter strength strength (per- (per- F.) (11111.) (K s.i.) (K s.i.) cent) cent) yield strength.

TABLE IV.-IN100 Tensile properties at 1,200 F.

Grain Reduce size Elongation of Compact diam- Yield Tensile tion area temperature eter strength strength (per- (per- F.) him.) (K s.i.) (K s.1.) cent) cent) sively toward the gamma prime solvus temperature of about 2225 F.

TABLE V.-ASTROLOY Rupture time Grain 1,400 F./

size 85,000

Compact temperadiameter p.s.i. ture F.) 04m.) (hour) Table V shows for Astroloy that if compacts of Astro- 10y are heated at temperatures above the gamma prime solvus temperature of 2050 F. for the alloy, the creeprupture strength increases significantly, particularly when the compact is heated to a temperature of 2275 F.

What is claimed is:

1. A method for producing powder metal articles from superalloy powder having therein a dispersion of gamma prime particles and an average grain diameter of 20 micron maximum, said method comprising heating a charge of said powder to a temperature of at least a selected compacting temperature but below the gamma prime solvus temperature of said superalloy powder and isostatically compacting said charge to a density of at least by the application of fluid pressure prior to said charge cooling to a temperature below said selected compacting temperature.

2. The method of claim 1 wherein said superalloy powder is placed in a container for heating and after said heating said powder-filled container is placed in a fluid pressure vessel for compacting.

3. The method of claim 1 wherein said selected compacting temperature is not lower than 400 F. below said gamma prime solvus temperature.

4. The method of claim 1 wherein said charge is hot worked after compacting at a temperature below the gamma prime solvus temperature.

5. The method of claim 1 wherein said charge after compacting is heated at a temperature below the gamma prime solvus temperature, whereby the yield strength is increased.

6. The method of claim 1 wherein said charge after said compacting is heated at a temperature above the gamma prime solvus temperature, whereby the creep and creep-rupture strength are increased.

7. The method of claim 1 wherein said compacting is by the application of fluid pressure within the range of 1,000 to 50,000 p.s.i.

8. The method of claim 1 wherein said powder is not larger than minus 30 mesh.

9. The method of claim 1 wherein said powder charge is compacted to an intermediate density prior to compacting to a density of at least about 95%.

10. The method of claim 1 wherein said powder charge is outgassed prior to compacting to a density of at least about 95% 11. The method of claim 2 wherein said container is gas tight and has an inert atmosphere therein during said heating.

12. A method for producing powder metal articles from superalloy powder having therein a dispersion of gamma prime particles and an average grain diameter of 20 micron maximum and an average powder size of minus 30 mesh maximum, said method comprising placing a charge of said powder in a container, evacuating said container, introducing an inert gas to said container, heating said charge to a temperature not lower than 400 F. below the gamma prime solvus temperature of said superalloy powder, but below said gamma prime solvus temperature, introducing said heated, powder-filled container to a fluid pressure vessel for compacting and compacting said powder charge to a density of at least 95% by increasing the fluid pressure within said vessel to a level within the range of 1,000 to 50,000 p.s.i.

13. The method of claim 12 wherein said charge after said compacting is reheated to a temperature between 1600 F. and the gamma prime solvus temperature and hot worked while at a temperature this range.

14. The method of claim 13 wherein said compacted charge after hot working is heated to a temperature above the gamma prime solvus temperature, whereby the creep and creep-rupture strength is increased.

15. The method of claim 13 wherein said charge after hot working is heated to a temperature below the gamma prime solvus temperature, whereby the yield strength is increased.

References Cited Waters et al.: Evaluation of Two Nickel Base Alloys, Alloy 713C and NASA TAZ-SA, Produced by Extrusion of Prealloyed Powders, NASA TN D-5248, May 1969.

Dumond, T. C.: Superplasticity Pulls Ahead in Metal Forming, Iron Age, Nov. 18, 1971, pp. -57.

Kelso T. D.: The Advent of Superplastic Superalloys, Manufacturing Engineering and Management; vol. 64, No. 5, May 1970, pp. 55 and 56.

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 

